The invention provides an ultra-narrow line width ring cavity laser based on parallel feedback, comprising an Erbium-doped optical fiber amplifier (EDFA), an F-P cavity filter, a single mode optical transmission medium, a multi-mode optical transmission medium, a 90:10 coupler and an isolator, wherein the EDFA is in connected series with the F-P cavity filter, the isolator, the multi-mode optical transmission medium and the 90:10 coupler by the single mode optical transmission medium so as to form a ring cavity; and the through end of the 90:10 coupler is connected into the ring cavity, and the coupling end thereof serves as an optical output port. According to the ultra-narrow line width laser of the invention, an optical transmission structure composed of the single mode optical transmission medium and the multi-mode optical transmission medium can be used for compressing the line width with no need of increasing the Erbium-doped optical fiber length, thus the defects of complicated structure and expensive cost of the existing ultra-narrow line width are solved, further the line width is greatly compressed, and the ultra-narrow line width laser output can be realized.